In recent years, particulates contained in exhaust gases that are discharged from internal combustion engines of vehicles, such as buses, trucks and the like, and construction machines have raised serious problems since those particulates are harmful to the environment and the human body.
For this reason, there have been proposed various ceramic filters that allow exhaust gases to pass through porous ceramics and collect particulates in the exhaust gases to purify the exhaust gases.
With respect to the ceramic filter of this type, conventionally, as indicated by a honeycomb filter 10 in FIG. 1, there have been proposed those having a structure in that a sealing material layer 14 is formed on a circumference of a columnar body 15 made of cordierite or the like, which has a number of through holes 11 placed in parallel with one another in the length direction with wall portion 13 interposed there between. In this honeycomb filter 10, wall portion 13 that separates the through holes 11 from each other functions as a filter (for example, see J UM Kokai Hei 7-183).
In other words, the columnar body 15 is constituted by a single sintered body, and as shown in FIG. 1(b), each of the through holes 11 formed in the columnar body 15 is sealed with a plug 12 at either of ends of its exhaust gas inlet side and outlet side, so that exhaust gases that have entered one through hole 11 are discharged from another through hole 11 after having always passed through wall portion 13 that separates the through holes 11 from each other.
Here, the sealing material layer 14 is formed in order to reinforce the circumference of the columnar body 15 or improve the heat-insulating property of the honeycomb filter 10.
Moreover, in recent years, in place of the above-mentioned honeycomb filter made of cordierite, honeycomb filters, made of a porous silicon carbide sintered material, have often been used because this material has advantages such as superior heat resistance, mechanical strength, collecting efficiency and the like, as well as chemical stability and a small pressure loss.
With respect to the honeycomb filter made of the porous silicon carbide sintered material, as indicated by a honeycomb filter 20 shown in FIG. 2, a honeycomb filter having the following structure has been proposed: a plurality of porous ceramic members 30 made of silicon carbide are combined with one another through sealing material layers 23 to form a ceramic block 25, and a sealing material layer 24 is formed on the circumference of this ceramic block 25 (for example, see JP Kokai 2001-162121). Moreover, as shown in FIG. 3, each porous ceramic member 30 has a structure in that: a number of through holes 31 are placed in parallel with one another in the length direction; and partition wall 33 that separates the through holes 31 from each other functions as a filter.
In other words, as shown in FIG. 3 (b), each of the through holes 31 formed in the porous ceramic member 30 is sealed with a plug 32 at either of ends of its exhaust gas inlet side or outlet side, so that exhaust gases that have entered one through hole 31 are discharged from another through hole 31 after having always passed through partition wall 33 that separates the through holes 31 from each other.
Here, the sealing material layer 24 is provided so as to prevent exhaust gases from leaking through the circumference of each ceramic block 25 when the honeycomb filter 20 is installed in an exhaust passage of an internal combustion engine.
The honeycomb filter 10 or the honeycomb filter 20 having the above-mentioned structure is installed in the exhaust passage in the internal combustion engine, so that particulates in the exhaust gases discharged from the internal combustion engine are captured by the wall portion 13 or the partition wall 33 when passing through the honeycomb filter; thus, the exhaust gases are purified.
In particular, since the honeycomb filter 20 shown in FIG. 2 has superior heat resistance and provides easy regenerating processes and the like, it has been applied to various large-size vehicles and vehicles with diesel engines.
In this arrangement, however, since the sealing material layer of the conventional honeycomb filter contains a comparatively large amount of organic components, in the case of a honeycomb filter in which sealing material layers account for a great ratio, it contains a large amount of organic components.
Moreover, the sealing material layer is not made into a completely dense body, and exhaust gases are slightly allowed to flow into the layer. For this reason, in the case where such a honeycomb filter, which has sealing material layers containing a large amount of organic components as described above, is installed in an exhaust passage in a vehicle such as a large-size vehicle, a vehicle having a diesel engine and the like, and used therein, the organic components are discharged outside through exhaust gases that is flown into the sealing material layer, resulting in a great increase in the amount of the organic components (HC) of exhaust gases.
Moreover, there have been also proposed honeycomb filters which support a catalyst in order to decompose and eliminate organic components in exhaust gases to be discharged outside. However, conventional honeycomb filters have failed to sufficiently decompose and eliminate the organic components by using the above-mentioned catalyst.